R1:e9693f25f89e

Move event framework from Phabricator to libphutil

Summary:
Move event framework from Phabricator to libphutil so it can be used in other
phutil projects, such as Arcanist.

Test plan:
Use along with path to libphutil, events should work as expected.

Reviewers: epriestley

Differential Revision: 1098